The present invention relates to communications networks that include satellite links. More specifically, the invention relates to a method that allows a communications satellite to substantially increase the number of information channels it can process without a corresponding increase in spacecraft hardware, weight, or power.
Modern communications networks carry staggering amounts of information, typically divided for transmission purposes into individual data channels. Whether the data channels carried by the communications network have their origin in the telephone system, television stations, or other source, these data channels are often combined into a single data stream. One common link in the communications network handling the data stream is a communications satellite.
A single satellite may have, for example, 30 or 40 uplink transponders (essentially receive antennae), each able to accept a data stream with a bandwidth of 250 MHz. The resultant uplink data path would then have a capacity of 8 to 10 gigabits per second. Where a satellite is a link in the communications network, Customer Premises Equipment (CPE) may combine the data channels into a single data stream, as well as encode, modulate, and transmit the uplink data stream to the satellite.
Because the uplink data stream sent to the satellite is susceptible to numerous sources of interference that can corrupt the uplink data stream, the CPE encodes the data stream with error protection codes. The first code the CPE applies is typically a block code. The block code essentially adds parity bits to each predefined number of bits in a data channel(a block). The block encoded data channels are then further encoded with a convolutional code to reduce the bit error rate (BER) to a tolerable level (BER is the ratio of incorrectly received bits to the total number of received bits). The sequence of coding described above is often referred to as "concatenated coding".
There are difficulties with concatenated coding, however. By design, the result of the convolutional coding is that the bits in the data channel become scrambled and interleaved among all the other bits in the data channel (including the block code bits). That is, they are no longer in order. As a result, the satellite must convolutionally decode each data channel recovered from the uplink data stream before the bits in each individual data channel can be recovered. The end result is an extremely complicated processing path for data channels in the satellite.
The processing path for the uplink data streams through a typical satellite include stages for receiving, decoding, switching, re-encoding, and transmitting downlink data streams to their destinations. The switching stage separates individual data channels from the uplink data streams and combines them with other data channels to form a composite downlink data stream that will be sent through a downlink antenna. A satellite characteristically uses a convolutional decoder followed by a block decoder to extract each data channel in the uplink data stream. Today, tens of thousands of channels may compete for services in an uplink data stream to be processed by the communications satellite. A single satellite would require enormous amounts of space, weight, and power to convolutionally decode each channel individually.
Increasing the size, weight, and onboard power of a satellite so that it can decode more data channels drives up the cost of the satellite dramatically. Not only does the satellite itself become more expensive because of the additional decoder circuitry and solar panels used to provide onboard power, but it also costs more to launch the satellite because larger rockets using more propellant are required to put the satellite into orbit.
The problem becomes even more significant when the highly complex encoding schemes employed by modern communications techniques are considered. One example of such a technique is Code Division Multiple Access (CDMA). This technique uses sophisticated encoding which generates massive amounts of data for the communication network to carry. CDMA transmissions to a satellite may, for example, divide the 250 MHz transponder bandwidth into smaller bandwidths of approximately 12 MHz, each carrying data channels for dozens of users. In order for the satellite to decode the data channel for each user, the satellite would have to carry hundreds (perhaps thousands) of sets of heavy, complex decoding electronics, and generate enormous amounts of power. Thus, satellite size, weight, and power restrictions prohibit the satellite from handling the large numbers of data channels that modern communications techniques can generate.
Therefore a need is present in the industry for an improved communications network, which overcomes the disadvantages discussed above and previously experienced.